1. Technical Field
The present invention relates to a white ink composition used for ink jet recording.
2. Related Art
An ink jet recording apparatus is known which includes nozzles through which ink is ejected in the form of droplets. For creating desired images using an ink jet recording apparatus, a variety of ink jet recording inks have been used which contain different constituents depending on the applications.
A white ink composition used for recording an white image on a recording medium is one of the ink jet recording inks. In general, a white ink composition contains a white coloring material, such as silica, titanium oxide or any other metal oxide, or hollow resin particles. The white coloring material enables the recorded image to look white. For example, JP-A-2012-25886 and JP-A-2012-25884 each disclose an ink set including an ink composition containing a pigment and a polymer and a white ink composition containing hollow resin particles and a urethane resin. In these ink sets, the refractive indices of the polymer, the hollow resin particles, and the urethane resin are controlled so that the difference between the highest and the lowest refractive index of the three refractive indices is 0.1 or less. Consequently, the ink sets can form images having good fixity with little unevenness in gloss.
On the other hand, a different type of white ink composition is known which contains resin particles having a high refractive index, instead of the above-mentioned white coloring materials such as metal oxides or hollow resin particles. For example, JP-A-2012-251142 discloses a white ink composition in which resin particles whose molecule has a fluorene skeleton are dispersed in water. The use of this white ink composition allows the surface of the recording medium to be covered with a white image without being viewed through the image.
However, when titanium oxide is used as a white coloring material, the titanium oxide is liable to settle during storage due to the difference in specific gravity from the medium of the white ink composition, such as water or an organic solvent. On the other hand, when hollow resin particles as disclosed in the above cited JP-A-2012-25886 and JP-A-2012-25884 are used, the settling of the coloring material during storage is more alleviated, but insufficiently, than when titanium oxide is used.
If the white coloring material settles, the sediment of the coloring material hinders the ink from flowing and causes ejection failure. Also, the concentration of the coloring material in the ink is reduced and, consequently, the whiteness of the recorded image can be reduced undesirably.
If a pressure is applied to the hollow resin particles, the hollows of the particles can be crushed. The hollow resin particles are each defined by a shell, and the difference in refractive index between the shell and the internal hollow enables the recorded image to look white. If the hollows are crashed, the whiteness of the image is reduced.
One of the solutions of those problems is to use resin particles having a high refractive index as disclosed in JP-A-2012-251142 instead of titanium oxide or hollow resin particles. Such resin particles are expected to minimize the above problems.
Unfortunately, resin particles having a high refractive index as disclosed in JP-A-2012-251142 cannot be satisfactorily adhere to the recording medium in some cases, and accordingly, the rub fastness of the recorded image may be insufficient. For this measure, a material capable of increasing the fixity of the resin particles can be added to the ink to enhance the rub fastness. However, the whiteness of the image may be reduced depending on the material added.